The present invention relates, generally, to the field of apparatus and methods for use in optical telecommunication networks and, in its preferred embodiments, to the field of apparatus and methods for dynamically routing wavelength channels in optical fiber DWDM networks.
In the modern age of information exchange, many companies depend upon telecommunications networks to carry out their daily business and often rely upon telecommunications providers to supply a fast and reliable network with a very high bandwidth. For today""s large scale telecommunication applications, optical fiber dense wavelength division multiplexed (xe2x80x9cDWDMxe2x80x9d) networks appear to be a very good first generation solution which address and/or meet such requirements. The challenge, however, is to optimize the capabilities of optical networks to create a second generation network for use in the future.
The second generation of optical networks may use transparent optical routing of numerous wavelength channels. While it is desirable to route such wavelength channels entirely in the optical domain using integrated optics technology, such routing presents a number of technological challenges. One of the main challenges is that such routing may require the use of wavelength-selective filtering elements compatible with integrated optics. Unfortunately, such wavelength-selective filtering elements are, typically, not tunable over the entire wavelength range of interest. For instance, with the advent of optical DWDM, the number of wavelength channels requiring routing in a particular optical fiber application may be on the order of one hundred to a thousand, and the total optical bandwidth may range between ten nanometers and hundreds of nanometers. Building a tunable wavelength filter that can be tuned over such a wide DWDM bandwidth is difficult. Another technological challenge stems from the need to route such wavelength channels at high speeds on a microsecond or faster time scale. These requirements place great demands on any technology, and are difficult to achieve in concert.
Two important, xe2x80x9cbuilding-blockxe2x80x9d circuits for routing wavelength channels in second generation optical and/or DWDM networks are likely to be (1) the tunable add/drop, in which one of xe2x80x9cNxe2x80x9d incoming wavelengths is dropped, and (2) the 1xc3x97N tunable wavelength demultiplexer, in which xe2x80x9cNxe2x80x9d wavelengths on an input channel are separated into xe2x80x9cNxe2x80x9d independent output channels, as shown in FIG. 1 (alternatively, the demultiplexer of FIG. 1 may be reconfigured to drop multiple wavelengths onto a single output optical fiber by using additional tunable wavelength filters, as depicted in FIG. 2). Due to reciprocity, these circuits may be used in reverse to perform wavelength add and Nxc3x971 wavelength multiplexing, respectively. In these circuits, 1 to xe2x80x9cNxe2x80x9d tunable wavelength filters are used to selectively xe2x80x9cdropxe2x80x9d, or direct, a selected wavelength channel to an output optical fiber. Each such tunable wavelength filter should be tunable over all wavelength channels.
Another important circuit for routing wavelength channels in second generation optical and/or DWDM networks is likely to be the Nxc3x97N wavelength routing switch circuit illustrated in FIG. 3. This circuit takes xe2x80x9cNxe2x80x9d inputs, each with xe2x80x9cNxe2x80x9d wavelengths, and routes one wavelength from each input to each output. Each output receives all xe2x80x9cNxe2x80x9d wavelengths, with each wavelength originating from a different input. As noted above, it is desirable for each of the N2 wavelength filters to have the ability to independently access all xe2x80x9cNxe2x80x9d wavelength channels.
Many of the prior art tunable filter technologies suitable for dense integration on an optical chip cannot tune over the full DWDM bandwidth, thus making implementation of the above-described circuits (and a variety of other circuits) impractical. For instance, arrayed waveguide gratings, acousto-optical and electro-optical tunable filters, and Mach-Zehnder interferometer techniques are so limited. Similarly, while being suitable for dense integration, neither the traveling wave optical microcavity filter nor longitudinal Bragg gratings can tune over the full DWDM bandwidth.
The traveling wave optical microcavity filter, which includes a tuned optical cavity, has resonances that allow the transfer of specific wavelengths from an input optical channel to an output optical channel. The tuned optical cavity supports whispering gallery modes which behave very similarly to the longitudinal modes of a linear Fabry-Perot type cavity. Fabry-Perot type resonators may be implemented with optical fiber or integrated onto an optical chip using reflective interfaces or longitudinal Bragg gratings. The length of the cavity determines the resonance wavelengths; which are the wavelengths that pass from the input channel through the cavity to the output channel with high efficiency. These wavelengths, or frequencies, given by       v    i    =      i    ⁢          xe2x80x83        ⁢          c      nL      
are periodic, with the period being given by the cavity free-spectral range (xe2x80x9cFSRxe2x80x9d), which is approximately       FSR    =                  Δ        ⁢                  xe2x80x83                ⁢        v            =              c        nL              ,
where xe2x80x9ccxe2x80x9d is the speed of light, xe2x80x9cnxe2x80x9d is the effective index of the cavity mode, and xe2x80x9cLxe2x80x9d is the round trip path length through the cavity. In DWDM systems, it is generally beneficial to have xcex94xcexd be greater than the total optical bandwidth, which is computed from the number of wavelength channels multiplied by the channel spacing. By doing so, a single wavelength channel may be operated upon without interference from other channels. Another condition that must be met is to have the resonance frequency passband, given by the expression             δ      ⁢              xe2x80x83            ⁢      v        =          v      Q        ,
where xe2x80x9cxcexdxe2x80x9d is the resonance frequency and xe2x80x9cQxe2x80x9d is the quality factor of the cavity (i.e., which is related to the losses in the cavity), be approximately equal to the wavelength channel spacing xcexdch.
Additionally, it is desirable to have the ability to tune the resonance frequency by one free-spectral range, so that all wavelength channels may be operated upon by a single cavity. Such tuning may be achieved by varying the index of refraction. To tune over the entire free-spectral range by changing the index of refraction requires that       Δ    ⁢          xe2x80x83        ⁢    n    =                    λ                  2          ⁢          L                    ⁢              xe2x80x83            ⁢      or      ⁢              xe2x80x83            ⁢                        Δ          ⁢                      xe2x80x83                    ⁢          n                n              =                  Δ        ⁢                  xe2x80x83                ⁢        v            v      
be achieved. For more general situations in which the tuning range is less than the free-spectral range, the condition xcex94xcexd/xcexd=xcex94n/n is still valid, where xcex94xcexd now represents the tuning range. For a number of tuning mechanisms, such as the electro-optic effect and the thermo-optic effect, the maximum achievable fractional index change, xcex94n/n, is of the order 0.01, meaning that the maximum cavity free-spectral range over which full tuning can be performed is xcex94xcexd≈0.01xcexd≈2 THz, which is much smaller than the optical bandwidth of interest such as that made available, for example, by optical fiber amplifiers, and therefore smaller than the total bandwidth that may be used by high capacity DWDM networks.
Alternatively, the length of the cavity could be changed by the amount                     Δ        ⁢                  xe2x80x83                ⁢        L            L        =                  Δ        ⁢                  xe2x80x83                ⁢        v            v        ,
but again, large amounts of change, such as provided by the piezoelectric effect, are difficult to achieve. Utilization of both refractive index and cavity length changes may increase the tuning by about a factor of two, but such an increase may still not be enough to cover the desired wavelength range. However, it should be noted that MEMS type devices with moving parts may achieve this goal, but may be very difficult to stabilize to a specific wavelength channel, as a positioning accuracy of             δ      ⁢              xe2x80x83            ⁢      L              Δ      ⁢              xe2x80x83            ⁢      L         less than                     δ        ⁢                  xe2x80x83                ⁢        v                    Δ        ⁢                  xe2x80x83                ⁢        v              ⁢          xe2x80x83        ⁢    or    ⁢          xe2x80x83        ⁢                  δ        ⁢                  xe2x80x83                ⁢        L            L         less than             δ      ⁢              xe2x80x83            ⁢      v        v    ≈      10          -      4      
must be attained, where xe2x80x9cxcex4Lxe2x80x9d is the necessary positioning accuracy. Therefore, these refractive index and length change considerations make it very difficult for a single traveling wave optical microcavity filter to be tunable over all wavelength channels.
Similar to traveling wave optical microcavity filters and as noted above, prior art longitudinal Bragg gratings, which may be fabricated in an optical fiber or waveguide on an integrated optical chip, also cannot tune over the full DWDM bandwidth. A Bragg grating strongly reflects wavelengths that satisfy the condition             λ      i        =                                        2            ⁢            n            ⁢                          xe2x80x83                        ⁢            Λ                    i                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  v          i                    =              i        ⁢                  c                      2            ⁢            n            ⁢                          xe2x80x83                        ⁢            Λ                                ,
where xe2x80x9cxcex9xe2x80x9d is the grating spacing, xe2x80x9cnxe2x80x9d is the refractive index, and xe2x80x9cixe2x80x9d is an integer. Bragg gratings are, typically, fabricated such that the grating spacing is one-half the wavelength (i.e., i=1), equal to the wavelength (i.e., i=2), or three-halves the wavelength (i.e., i=3). The free-spectral range, FSR, can be written as       Δ    ⁢          xe2x80x83        ⁢    v    =            "LeftBracketingBar"                        v                      i            +            1                          -                  v          i                    "RightBracketingBar"        =                  c                  2          ⁢          n          ⁢                      xe2x80x83                    ⁢          Λ                    .      
In the communications bands, for such grating spaces, the free-spectral ranges would be approximately 200 THz, 100 THz, and 67 THz, respectively. Therefore, the free-spectral range of Bragg grating type filters, typically, exceeds the DWDM spectrum.
The frequency tuning range, xcex94xcexd, of a Bragg grating may be written in terms of a refractive index change or grating spacing change                     Δ        ⁢                  xe2x80x83                ⁢        v            v        =                  -                              Δ            ⁢                          xe2x80x83                        ⁢            n                    n                    =              -                              Δ            ⁢                          xe2x80x83                        ⁢            Λ                    Λ                      ,
where, again, the same limitations on refractive index and/or cavity length change apply. Therefore, the total tuning range may only be of the order of 2 THz, which is not sufficient to tune over the full DWDM bandwidth.
Typical optical DWDM systems operate with a 100 GHz or, more recently, a 50 GHz channel spacing. It is expected that such systems may ultimately employ hundreds of wavelengths in each of the xe2x80x9cCxe2x80x9d and xe2x80x9cLxe2x80x9d bands, as the width of each band is of the order of 10 THz. In order to implement filtering elements tunable over the entire range of one such band, fractional changes of approximately 0.05 in filter parameters are needed. Using presently available materials, a more practical fractional change of about 0.003, for example, gives a tuning range of about 600 GHz, or about 6 (for 100 GHz spacing) or 12 (for 50 GHz spacing) wavelength channels.
Therefore, there exists in the industry, a need for optical wavelength routing circuits having wavelength-selective filtering elements compatible with integrated optics and operable at high speeds which are tunable over all DWDM channels, and which address these and other related, and unrelated, problems.
Briefly described, the present invention comprises apparatus and methods, tunable and operable over all DWDM wavelength channels, for optically routing such channels. According to the present invention, incoming wavelength channels are separated into a plurality of sub-groups having a smaller optical bandwidth. Wavelength channels within each sub-group are then acted upon independently by a filter, or switch, which is tunable and operable over the reduced bandwidth of each sub-group. Where necessary, the sub-groups are then recombined to form a desired output composite channel.
In the preferred embodiments described herein, the present invention is embodied in two classes of dynamic wavelength routing circuits, including, for example and not limitation, 1xc3x97N and Nxc3x97N circuits. In the first class of circuits, the filter free-spectral range (and, therefore, the necessary tuning range) is a fraction of the full DWDM bandwidth, and is represented as the bandwidth of a sub-group of wavelength channels. These circuits require that the wavelengths be divided into sub-bands by a DWDM demultiplexer and are more suitable for filters whose free-spectral range is typically less than the DWDM bandwidth (e.g., resonant cavities), such that tuning within each sub-group may be performed with filters having a single filter design. In the second class of circuits, the circuits include filters having a free-spectral range greater than the full DWDM bandwidth, but still have a limited tuning range (e.g., the filters include Bragg gratings and resonant cavities of small size). The filters of circuits in the second class must tune over different wavelength ranges to cover the entire DWDM bandwidth. Note that most types of circuits devised for the first class may be implemented by the second class of circuits, and vice versa. Circuits which represent a hybrid between the two classes are also described herein, and may provide additional flexibility not offered by circuits of the first or second classes alone.
It should be understood that while the present invention is described herein in the form of particular illustrative circuits, the present invention has applicability to a plurality of other circuits as well. Also, as described herein, practical filters may be tunable, for example, over six to twelve wavelength channels, while there may be a hundred or more channels, for example, which require routing. Therefore, use of the present invention for routing the entire number of possible channels may require division of the DWDM bandwidth into ten to twenty sub-groups of twelve to six wavelengths each. However, for simplicity of description and illustration, the circuits herein are described herein with reference to a greatly reduced number of wavelength channels. Thus, it is important to note that the present invention is not limited by the artificial constraints imposed for purposes of the description herein.
Accordingly, it is an object of the present invention to provide an apparatus and/or method for separating an incoming optical waveform into a plurality of sub-groups of wavelength channels having a smaller optical bandwidth than the incoming optical waveform.
Another object of the present invention to provide an apparatus and/or method for separating an incoming optical waveform into a plurality of sub-groups of wavelength channels having a smaller optical bandwidth than the incoming optical waveform and for recombining the sub-groups to form a desired output channel.
Still another object of the present invention to provide an apparatus and/or method for separating an incoming optical waveform into a plurality of sub-groups of wavelength channels having a smaller optical bandwidth than the incoming optical waveform and for tuning the channels of each sub-group.
Still another object of the present invention to provide an apparatus and/or method for cumulatively tuning the entire DWDM bandwidth by tuning a plurality of sub-groups of wavelength channels having a smaller optical bandwidth.
Other objects, features, and advantages of the present invention will become apparent upon reading and understanding the present specification when taken in conjunction with the appended drawings.